Dies used to extrude a foamed thermoplastic polymer in a long-barrel extruder (barrel length "L":barrel diameter "D" at least 24:1) to form an article of arbitrary length and cross-section are not subjected to the high forces to which dies are subjected when either a solid or a molten metal is extruded. In the latter "metal extrusion dies", the major consideration is strength which must be sufficient to withstand forces high enough to distort or crack a die. Particularly in "foam extrusion dies" to form an extrudate having a density of less than about 0.48 gm/cm.sup.3 (30 lb/ft.sup.3), the major consideration is control of a mixture of particulate thermoplastic and blowing agent in the melt composition, temperature and pressure in the barrel of an extruder, the rate at which the extrudate is formed relative to the capacity of the barrel, and control of the conditions to which the melt is subjected due to the geometry of the die, all towards the specific goal of producing a smooth surface on an extrudate with reliably reproducible dimensions. Because it was believed that narrowly controlled laminar flow was essential to produce an acceptable extrudate, a conventional die is provided with a tapered land, preferably tapered so as not to change the direction of flow of the melt. It was also believed that a land length was to be short enough to minimize pressure drop and premature foaming but long enough to withstand system pressures and form the desired profile thoroughly.
The foregoing is stated in U.S. Pat. No. 5,654,346 to Halberstadt et al who found that a flat plate is an effective substitute for a die with a tapered land particularly for extrusion of a foam having a density of less than about 0.32 gm/cm.sup.3 (20 lb/ft.sup.3) derived from a "soft" thermoplastic resin, namely one having a Shore A hardness of less than 72. The evidence is that a soft thermoplastic resin is effectively extruded through an orifice profiled in a flat plate, but a "hard" thermoplastic resin having a Shore A hardness greater than 72, requires a die with a tapered land.
It is desirable to provide a die with a design which produces a substantially smooth-surfaced extrudate having an accurately defined cross-section irrespective of the hardness of the thermoplastic resin, and particularly of a resin which is a vulcanizable blend of from 10 to 50 parts by weight ("wt"), preferably 20 to 50 parts by wt, of a crystalline a-olefin polymer having a repeating unit with from 2 to 4 carbon atoms, and from 90 to 50 parts by weight, preferably 80 to 50 parts by wt of a rubber copolymer, as exemplified by commercially available Santoprene.RTM. resins.
Theoretical considerations in TPV (thermoplastic vulcanizate) extrusion are set forth in "Analysis of TPV Extrusion Chemical Foaming Process" by Wang et al in a paper presented at the Fall Meeting in Proceedings of the American Chemical Society Division of Polymeric Materials: Science and Engineering on Aug. 23-28, 1998. It is stated that "Premature foaming inside the die will destroy or damage the cell structure resulting in higher foam density and bad foam surface. In principle, the bubble growth process should be postponed to a point as close to the die exit as possible. This can be achieved by designing the die with a sharp converging angle and short land length."
It is particularly desirable to extrude a weatherseal having a low-friction surface, irrespective of its roughness or smoothness (Ra) as measured with a model EMD-0400-W5 Surfanalyzer with a stylus having a radius 0.0001" and with a stylus force of 200 mg, for use in an application where a glass surface is moved back and forth over the extrudate. Such weatherseal against which the window glass of an automobile door slides up and down is generally formed of a soft synthetic resinous vulcanizate which is provided with a fiber nap such as a nylon nap on the glass-abutting surface along which the glass is to be slidably translated. An alternative is to adhesively secure a low-friction tape of a synthetic resin such as polytetrafluoroethylene (PTFE). As stated in the '655 patent, neither alternative is satisfactory; fiber napping is complicated and the nap is short-lived; and coating the appropriate surfaces with adhesive to which the PTFE tape is to be attached is both complicated and unsatisfactory as the tape is to be secured on a curved surface and around corners.
The crystalline polyolefin is either a homopolymer of an .alpha.-olefin having from 2 to 4 carbon atoms, or a copolymer of two or more of such .alpha.-olefins. Preferred are polyethylene (PE) or polypropylene (PP), although copolymers of either ethylene or propylene with a minor amount of a higher alpha olefin can be used. Polypropylene is most preferred. The crystalline nature of the polyolefin provides desirable properties such as high tensile strength and thermoplasticity to the blends,
Rubbers useful in the foamed blends include butyl rubber, halobutyl rubber, EPDM (ethylene/propylene/diene rubber) and EPR (ethylene/propylene rubber) rubber, acrylonitrile/butadiene rubber (NBR) and natural rubber. Combinations of two or more rubbers of different types can also be used. Thermoplastic elastomers which can be successfully foamed by the process of the invention are described in the following U.S. patents, the disclosures of which are herein incorporated by reference: U.S. Pat. Nos. 4,104,210; 4,130,534; 4,130,535; 4,299,931; and 4,311,628; inter alia. Also useful are blends of crystalline polyolefin plastics and partially cured rubbers, such as those described in U.S. Pat. Nos. 3,806,558 and 3,862,056, and blends of crystaline polyolefins and uncured EPR or EPDM rubber. The term "elastomer" is used herein to refer to a vulcanized blend of polyolefin and rubber which may be formulated to exhibit varying degrees of elasticity such that a test strip 2.5 cm wide and 2.5 mm thick may be stretched in the range from about 5% to 100% of its initial length and still return to it; further, such vulcanized elastomer is necessarily thermoplastic and re-processable. Typically, a relatively soft, fiexible product is desired, so the softer grades of elastomer are preferred, which have high ratios of rubber to polyolefin, such as from 65:35 up to 85:15. The elastomers can optionally contain other ingredients, including oils, waxes, fillers, colorants, antidegradants and the like.
It is preferred that the rubber be at least partially cured, and more preferred that it be fully cured without using a sulfur curative. Preferred curatives for the elastomers of this process are peroxide curatives or phenolic resin curatives, with the latter being especially preferred. Zinc oxide is an effective curative for halobutyl rubbers. The phenolic curative svstem includes a methylolphenolic resin and an activator. The activator is preferably a Lewis acid. Thermoplastic elastomers of the preferred type are described in detail in U.S. Pat. No. 4,311,628.
Commercial thermoplastic elastomers which are effective in the process of the invention include those which are manufactured and sold by Advanced Elastomer Systems, L.P., under the registered trademarks TREFSIN, SANTOPRENE, GEOLAST, VYRAM, and TPR; by DSM under the registered trademarks SARLINK; and by others.
Also effective when dynamically vulcanized with the foregoing TPEs, are rubbery polymers such as copolymers of ethylene and vinyl acetate; and copolymers of a diene and a vinyl aromatic such as copolymers of styrene and butadiene, as exemplified by commercially available Kraton.RTM. rubbers. Such rubbery polymers may be tumble-blended with a chosen TPE in an amount in the range from about 5% to 20% by wt of the tumble-blended mixture.
The foregoing elastomers are used to form the extrudates in the '346 and the '889 patents inter alia, but the conditions for extrusion, particularly the rate of extrusion (mass flow of melt) relative to the capacity of the barrel of the multi-zone extruder, are not specified; nor is the condition of the surface, though stated as being smooth. Smoothness is of particular importance in weather stripping and window-glass both of which require a smooth, non-undulating or non-wavy surface; a wavy surface does not provide a seal against incoming air or water. A desirably smooth surface having a roughness less than 300 Ra is not obtained with a foamed extrudate of the above composition having a density less than 30 lb/ft3 because it is conventionally produced in a multi-zone extruder with a temperature profile described in U.S. Pat. Nos. 4,898,760; 5,788,889 and the '346 patent inter alia, that is, the temperature in the feed zone is lower than that in the discharge zone near the die (referred to as a "low-to-high temperature profile", shown in Table I of the '346 patent to be 160.degree. C. (320.degree. F.) and 185.degree. C. (365.degree. F.) respectively. As is well known, a higher melt temperature near the feed hopper than the temperature near the die, tends to plug the barrel due to build-up of melt (see Encyclopedia of Polymer Science and Engineering Vol 1, pg 54) prematurely plugging the throat of the machine.
Further since surface is to a large extent a function of composition and density of the extrudate, it is essential to specify the composition which provides a surface with particular characteristics. A smooth bearing-surface, sufficiently smooth to allow the glass of an automobile window to be retracted into the door and raised into a position sealing the window, all the while in contact with the weather strip, cannot be produced directly by extruding a foam having the composition specified herein. Therefore, as shown in U.S. Pat. Nos 5,343,655; 5441,685 and 5,447,671 assigned to Tokiwa Chemical Industries, the bearing-surfaces of the weather strip are coated with a contacting layer which provides the necessary surface. Though superposing a contact layer on only the bearing surface avoids modifying a major portion of the surface of the weatherseal, thus maintaining the desirable softness of the extrudate, this is not an easy task. Since it is evident that coating the entire surface with a molten polyolefin will provide a smooth and hard bearing surface, the problem is that a coating thick enough to provide a smooth layer on a conventionally "smooth" foamed extrudate such as is used in the aforesaid patents, is typically so thick that it interferes with the flexibility and softness of the weatherseal. The smooth non-porous skin of the novel extrudate allows a molten polyolefin to be co-extruded over the entire surface of the extrudate in so thin a coating as not to adversely affect either the softness or flexibility of the weather seal, yet allow the glass to travel along the surface of the extrudate with desirable ease. Further, the dense skin allows the production of a foamed extrudate which is strong and durable despite having a density 70% less than that of the solid unfoamed elastomer.